1. Field of the Invention
The invention is directed to wire grid polarizers and particularly a polarizer for the infrared portion of the electromagnetic spectrum usable with an infrared acousto-optic tunable filter.
2. Description of the Prior Art
An acousto-optic tunable filter operates through the interaction of high frequency acoustic waves with light waves in a suitable crystal. In certain birefringent optical materials, a light beam propagating as an e-ray, can, under certain conditions, be converted into an o-ray by interaction with, and diffraction from, an acoustic wave propagating in the same medium. This phenomenon has been utilized in producing narrow band optical filters in which the peak transmission wavelength can be selected by properly choosing the frequency of the acoustic wave. Such filters have typically operated in the visible spectral region using collinearly propagating acoustic and light beams in selected oxide materials. It is also known that the acoustic wave can be launched in the acoustic medium non-collinearly with the light beam to achieve the same acousto-optic narrow band filtering.
The development of new efficient infrared acousto-optic materials such as thallium-arsenic-selenide (Tl.sub.3 AsSe.sub.3) as described in U.S. Pat. No. 3,792,287, the thallium-phosphorous-selenide per U.S. Pat. No. 3,929,970 and thallium-arsenic-sulfide per U.S. Pat. No. 3,799,659, all assigned to the assignee of the present invention, provides the possibility of operation over the near-to-medium infrared range of wavelengths from about 1.3 micrometers to about 16.0 micrometers. The acousto-optic interaction causes the "filtered" wavelengths of light to experience a 90.degree. polarization flip. Thus it has been the practice to utilize input and output polarizers with such acousto-optic materials to effectuate the filter capability. The polarizers have their axes crossed so that only appropriately polarized light reaches the acousto-optic material, and then only light which has undergone the 90.degree. polarization rotation is transmitted from the acousto-optic material through the system.
While a wide selection of low cost polarizers exist for the visible portion of the spectrum, most of these devices do not transmit well out to 16 .mu.m. Infrared transmitting materials which can be used for making prism polarizers are expensive. Additionally, the angular aperture of infrared prism polarizers tends to be much less than the allowable angular aperture of the acousto-optic device. While it is known to utilize prism type or pile-of-plate infrared polarizers, these prior art polarizers tend to limit the radiation throughput through the filter because such polarizers are only efficient for near normal incident light. These polarizers are also very expensive to fabricate and difficult to align with the rest of the light input and detection system with which the filter finds application.
It is also known that an infrared polarizer can be fabricated by providing an array of parallel, spaced conductors upon an infrared transmitting substrate with the spacing between adjacent conductors being less than the wavelength of the infrared light. As shorter wavelengths become of interest, the spacing between conductors is narrowed and in the range of a 1 .mu.m optical wavelength it becomes necessary to produce wire grids of at least 1000 wires per millimeter to achieve significant polarization of the optical beam.
It is the present practice to produce wire grid polarizers with wire densities greater than 1000 wires per millimeter on an infrared transmitting substrate. According to a first method, a wire grid polarizer is formed by ruling a finely spaced diffraction grating on a precisely polished zinc selenide (ZnSe) substrate. The ruled area is then coated with aluminum which is vacuum deposited at an oblique angle to form highly conductive wires at the peaks of the grooves. In a second method, a wire grid polarizer is formed by coating the substrate with photoresist and exposing it with a laser interference pattern. The etched sinusoidal photoresist profile is aluminized at an oblique angle to obtain the required array of conductors.
While the above described wire grid polarizers possess many desirable optical characteristics, they are plagued by their prohibitive cost and extreme fragility.
It is an object of this invention to provide a wire grid polarizer for use with an acousto-optic tunable filter in the infrared range.
It is also an object of this invention to provide a wire grid polarizer which is both inexpensive and much less susceptible to damage during handling.
It is another object of this invention to provide a wire grid polarizer ideally suited for use with a thallium-arsenic-selenide (Tl.sub.3 AsSe.sub.3) acousto-optic crystal.